Weight Watchers International Inc. said its chief executive had resigned and the company’s most-famous director, Oprah Winfrey, will help select his successor.

James Chambers, 58 years old, who has run the weight-loss company for the past three years, will leave at the end of the month. He will be succeeded on an interim basis by a three-person Office of the CEO while the board searches for a permanent leader. Mr. Chambers couldn’t...